1. Field of the Invention
This invention relates to the field of monitoring particular characteristics of coatings and films that have been deposited upon optical fibers, and specifically to monitoring the thickness and uniformity of such coatings and films that are electrically conductive.
2. Description of the Prior Art
It is well known in the art that the deposition of certain thin films, or coatings, on bare optical glass fibers effectively reduces water corrosion and other chemically induced stress corrosion of the fibers. In addition, thin films and coatings are effective in reducing light, attenuation through the fiber attributable to the absorption of hydrogen into the fiber from the environment. Such films or coatings are often referred to as hermetic coatings.
It is also common in the art to apply an additional outer protective coating over a previously deposited hermetic coating. This is to ensure that the hermetic coating is not damaged or portions inadvertently removed while the optical fiber is being handled or while the optical fiber is in service.
Hermetic coatings can be, and often are, categorized according to whether a coating is made of: an electrically conductive material, e.g., a material made predominately of carbon or metal; a semi-conductive material, e.g., silicon carbide; or a non-electrically conductive material, e.g., an inorganic material or a ceramic material If a particular hermetic coating is electrically conductive, or semi-conductive, the coating will consequently have a measurable linear electrical resistivity. The resistivity may be expressed in terms of ohms per centimeter, or other units that may be convenient.
It is a known concept in the art that the magnitude of the electrical resistivity is influenced by such factors as the thickness and/or the uniformity of the coating over a given length of the coated fiber. Subsequently it is known that as coating thickness increases, electrical resistance decreases. It is also known that a non-uniformity such as a bare section of an otherwise properly coated fiber would cause an increase in electrical resistance. Another example of coating uniformity affecting electrical resistance is a coating consisting of a mixture of a conductive material and a non-conductive material, wherein the coating thickness is acceptable, but the differing materials are improperly proportioned.
As previously known, and conceptually representative of similar methods for measuring the electrical resistivity of a conductive hermetic coating utilizes a pair of suitable contacts which are connected to a standard commercially available ohm-meter. The contacts are positioned at a predetermined distance from each other along the axis of the coated fiber to be measured. A measurement of the resistance between the points of contact is then taken and the value noted. If the hermetically coated fiber to be measured has an additional outer protective coating, a pair of razor blades can effectively be used as contact points. The razor blades are convenient for cutting through the outer protective coating while also providing electrical contact with the conductive hermetic coating below. An apparatus for carrying out this previously known method is illustrated in FIG. 1, wherein razor blades 70 and 72 are separated at a predetermined length by being fixed at opposite ends of an insulating block 74 and are respectively connected to ohm-meter means 78. Coated fiber to be measured 76, and razor blades 70 and 72, are then brought into contact with each other and the corresponding value of linear resistance is read from ohm-meter means 78.
After measuring the electrical resistance of a known length of a given coating, the thickness of a particular coating can then be empirically correlated with the measured resistance. This correlation is achieved by measuring the electrical resistance of a previously identified segment of a given coated optical fiber, and then measuring the actual thickness of the hermetic coating by measuring a cross-sectional sampling of that particular segment of coated fiber by electron microscopy, or by other methods. Thereafter, the correlation is carried out over a number of various coating thicknesses for a particular coating in order to set an acceptable range of resistivity that will indicate that coating thickness is meeting previously set production or quality assurance standards.
Correlation of coating uniformity can be carried out much the same way, except that a chemical analysis or a magnified visual inspection of the coating would be substituted for the measuring of coating thickness as set forth above.
A disadvantage with previously known methods of monitoring conductive hermetic coatings by electrical contact is that a portion of any protective coating present on the fiber must either be penetrated, mechanically removed, or chemically removed in order for electrical contact to be made with the hermetic coating underneath.
Another disadvantage with previously known methods of monitoring conductive hermetic coatings by electrical contact is that the section of hermetically coated optical fiber that is measured is usually rendered unusable by the physical contact between the electrical contact points and the hermetic coating. This is due to the fact that many optical fibers are very fragile and a nick or a scratch to or through the hermetic coating could produce a flaw in the form of a stress concentration in the fiber below. Thus, measuring the resistance of coatings at any place along the fiber other than end portions by previously known methods, necessitates undesirable splicing of the fiber to remove areas where flaws may have been created.
A further disadvantage with currently known methods of monitoring conductive coating thicknesses by electrical contact is that the electrical resistivity measurements can only be made on a time consuming static basis and not on a continuous/dynamic basis. In other words, the optical fiber being monitored must be stationary, or at least be moving very slowly, before a sample of that fiber may be removed and measurements be performed thereon. This precludes monitoring the thickness of the optical fiber while it is in motion such as on a high-speed production line. Therefore, there remains a need in the art for a quick and a non-destructive method for either statically or continuously monitoring coating thickness and/or coating uniformity without impeding the motion of the coated fiber.